Hypothalamic thermosensitivity and regulation of heat storage behavior in a day-active desert rodentAmmospermophilus nelsoni

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1. The characteristics of hypothalamic thermosensitivity ofAmmospermophilus nelsoni were determined at three ambient temperatures by manipulating the temperature of the hypothalamus with chronically implanted, water-perfused thermodes while continuously measuring metabolic rate (Figs. 1, 2, and 3). 2. Hypothalamic thermosensitivity as measured by the proportionality constant for the heat production response to hypothalamic cooling (αMHP) was extremely high in the 6 individuals studied, averaging −5.1 watts kg−1 °C−1 at aTa of 10°C, −5.4 watts kg−1 °C−1 at 20°C, and −7.6 watts kg−1 °C at 30°C (Table 1). 3. The threshold hypothalamic temperature for the metabolic heat production response to hypothalamic cooling decreased asTa increased a veraging 42.2 °C at aTa of 10 °C, 40.9 °C at aTa of 20 °C, and 39.1 °C at aTa of 30 °C (Table 1). 4. High hypothalamic temperatures (>40 °C) induced a long-lasting suppression of the level at whichTb was regulated (Fig. 4). This fall inTb was associated with a decrease in αMHP or the hypothalamic threshold temperature for the metabolic heat production response (Figs. 5 and 6). It is hypothesized that these changes in the hypothalamic regulator of body temperature resulting from high hypothalamic temperatures are the physiological bases of the alleged heat storage thermoregulatory behavior of this small, day-active, desert mammal. 5. A neuronal model of the CNS thermoregulatory system is presented (Fig. 7) which offers possible explanations for the two unusual features of the antelope ground squirrel's thermoregulatory system descriged in this study, namely the increase in the absolute value of αMHP asTa increases and the suppression ofTb, αMHP, and the thresholdThy for the metabolic heat production response following episodes of high hypothalamic temperature.